Unable to play MP3 or MP4 files in Rhythmbox or Movie Player

You need to install gstreamer plugins. To install it, run this command in your terminal (Ctrl+Alt+T):

sudo apt-get install ubuntu-restricted-extras

This will install the required plug-ins for playing MP3 and MP4 files.

By default, Ubuntu doesn't include these plugins, because they are non-free. However, you can install these plugins from the Ubuntu Software Center (search for mp3 plugins).

To access the Software Center, click on the Dash Icon (the topmost icon in the left hand side menubar that's displayed on the desktop) and type (search for)

Ubuntu Software Center

Left click on the icon to open Ubuntu Software Center.

If you want to install any other application, you could also use the Ubuntu Software Center.

For example, you could also install VLC media player (which is an excellent audio and video player) from the Ubuntu Software Center along with the MP3/MP4 plugins.

Or when this error is displayed, it also gives the option to download the plugins. Click it to download and install these plugins.

The link from the Ubuntu Website is https://help.ubuntu.com/community/RestrictedFormats/MP3.